Longitudinal measurements of cognition, brain glucose metabolism as determined with positron emission tomography (PET), and brain anatomy using magnetic resonance imaging, were conducted in five Down syndrome subjects initially older than 40 years of age. Changes in memory test scores over 5 years preceded the appearance of overt dementia in two subjects, indicating that a pre-dementia prodrome can be identified in subjects certain to develop Alzheimer type dementia. Predementia changes also could be identified by submitting older, non-demented subjects to audiovisual stimulation while measuring regional glucose metabolism with PET.